The present invention relates to a magnetic coating for magnetic recording media. More particularly, the present invention relates to a magnetic coating for magnetic recording media containing a copolymer comprising (A) vinyl chloride, (B) a vinyl carboxylate ester, (C) vinyl alcohol and (D) an unsaturated dicarboxylic acid and/or an unsaturated discarboxylic acid anhydride; a thermosetting resin; a magnetic powder; and an organic solvent.
In general, a magnetic recording media, such as a magnetic tape, a magnetic card and the like, is prepared by coating a substrate, such as polyester films, sheets and the like, with a magnetic coating which comprises a magnetic powder, a thermoplastic resin and a thermosetting resin, dissolved and dispersed in an organic solvent.
Vinyl chloride copolymers, vinylidene chloride copolymers, acrylic ester copolymers and methacrylic ester copolymers have been proposed as a thermoplastic resin which is suitable as substrate for the magnetic coating. Examples of such resins include vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol terpolymer, vinyl chloride-vinyl acetate-maleic acid terpolymer and the like. However, when the magnetic coating is prepared using these resins, the following are not satisfactory: miscibility of the magnetic powder with the thermoplastic resin; miscibility when a thermosetting resin, such as polyurethane resin, is added; affinity of the substrate to be coated; state of the resulting magnetic coated film; and physical properties of the magnetic coating. It has been difficult to satisfactorily provide all such required physical properties for a magnetic coating with only one resin.
For example, vinyl chloride-vinyl acetate copolymer has poor reactivity with the polyurethane resin. When the magnetic coating containing the vinyl chloride-vinyl acetate copolymer is coated on the substrate, the coated film is easily peeled off and impractical for the magnetic recording media. Although vinyl chloride-vinyl acetate-vinyl alcohol terpolymer has a good dispersibility for the magnetic powder and a good reactivity with the polyurethane resin, the use of the terpolymer as a starting material for the magnetic coating provides a coating having a short pot life, and therefore, an inferior workability results. When such magnetic coating is applied onto substrates, such as polyester sheets, films and the like, the viscosity of the coating may increase with the elapse of time. Accordingly, it is necessary to lower the viscosity of the coating by initially adding a large amount of the organic solvent when preparing the coating. Thus, it is impossible to prepare a large amount of the coating at one time.
In general, vinyl chloride-vinyl acetate-maleic acid terpolymer is restricted in a maleic acid content. If the maleic acid content thereof is too large, a clear solution cannot be obtained, and therefore, the content is restricted from about 1% to 2%. However, the use of such a vinyl chloride-vinyl acetate-maleic acid terpolymer containing such a small amount of maleic acid cannot sufficiently improve the dispersibility of the magnetic powder. Also, the vinyl chloride-vinyl acetate-maleic acid terpolymer has a high reactivity with polyurethane resin, and with other terpolymers, and therefore, the use of the terpolymer as a starting material for the magnetic coating provides a coating having a short pot life and inferior workability.